The Keystone Service is a 195 mile regional passenger train service from Amtrak between the Harrisburg Transportation Center in Harrisburg, Pennsylvania, and 30th Street Station in Philadelphia, running along the Philadelphia to Harrisburg Main Line. Most trains continue along the Northeast Corridor (NEC) to Penn Station in New York City.

The Philadelphia to Harrisburg Main Line is a rail line owned and operated by Amtrak in the U.S. state of Pennsylvania. This is the only electrified Amtrak line in the United States outside of the main line of the Northeast Corridor. The line runs from Philadelphia, where it meets the Northeast Corridor at Zoo Junction at milepost 1.9, west to Harrisburg, where electrification ends. The Main Line is part of the longer Keystone Corridor, which continues west to Pittsburgh along the Norfolk Southern Railway's Pittsburgh Line. This section is sometimes referred to as "Keystone East" and is part of Amtrak's Keystone Service.